In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
A typical industrial circuit breaker will have a continuous current rating ranging from as low as 15 amps to as high as several thousand amps. The tripping mechanism for the breaker usually consists of a thermal overload release and a magnetic short circuit release. The thermal overload release operates by means of a bimetallic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross-section and related elements are specifically selected for such current range resulting in a number of different current ranges for each circuit breaker. Electronic trip units are also used in some applications.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker. Again, electronic trip units can also be used.
Ratings of circuit breakers are continually increasing due to market driven requirements for space saving electrical equipment. As the ampere rating for a given circuit breaker frame size increases, space for wiring lugs within that circuit breaker becomes a premium. Lug size for attaching the various wires and cables is primarily driven by the wiring cable dimensions as defined in the National Electric Code or other country specific wiring standards or practices. Although this problem exists for all circuit breakers, it is especially acute for circuit breakers in the 100 amp to 125 amp range. In addition, the location of the wire lugs in the circuit breaker generally occupies the same relative space as the arc venting area near the main contacts. As the main contacts separate under an overload or short circuit condition, heat, gases, and arc by-products which are generated by the arcing in the arc chamber must vent out of the circuit breaker's housing. Such out gassing typically envelopes the wire lug and cabling near the arc chamber of the circuit breaker. The close proximity of the wire lug, and the load and line terminals with the contact of the circuit breaker create additional space limitations because of insulation requirements. Prior arrangements to address such problems include providing larger housing for the circuit breaker to accommodate the thicker insulations and larger wire lugs and cables. Prior arrangements also included requiring additional gas venting deflectors. Such prior arrangements are more expensive and complex in relation to the benefits sought and not as effective or reliable.
Thus, there is a need for a molded case circuit breaker having a wire lug/arc vent barrier for protecting a wire lug in a circuit breaker utilizing a stacked pole construction. There is also a need for a wire lug/arc vent barrier that will direct arc gasses and by-products around the wire lug and wire binding screw. There is further need for a wire lug/arc vent barrier that incorporates the functions of a lug barrier, arc chamber venting and line end insulation system in a single integral molded piece. There is further need for a wire lug/arc vent barrier for a molded case circuit breaker that allows the ampere rating of the breaker to be increased without increasing the overall size of the circuit breaker. There is additional need for a wire lug/arc vent barrier for a molded case circuit breaker that provides for easy assembly and mounting within the circuit breaker housing.